To reduce the emission of pollutants, in particular the emission of nitrogen oxides during the operation of combustion systems, exhaust systems of internal combustion engines in motor vehicles are equipped with catalytic converters. Using these, most of the hydrocarbons and carbon monoxide contained in the exhaust gas are burned. However, a large portion of harmful nitrogen oxides, which are discharged into the environment, remains in the exhaust gas when conventional catalytic converters are used.
The nitrogen oxide content in the exhaust gases can also be reduced by using reduction-type catalytic converters. Reduction of nitrogen oxides by adding reduction agents to an exhaust gas flow, also known as selective catalytical reduction (SCR), is known from European Patent Application No. EP 1 024 254.
The reduction agent quantity is determined here based on a load variable, e.g., injected fuel quantity and/or the engine speed, and at least one performance characteristic, e.g., the exhaust gas temperature upstream from the catalytic converter. Moreover, by using at least one characteristics map, the reduction agent quantity is adjusted as a function of at least one additional performance characteristic, e.g., the exhaust gas temperature downstream from the catalytic converter.
For this purpose, a temperature difference is formed between the actual temperature and the setpoint temperature of the exhaust gas downstream from the catalytic converter. Different characteristics maps, in which an adjusted reduction agent quantity is stored as a function of the engine speed and the injected fuel quantity, are provided for different temperature differences.
In order to take into account all occurring temperature differences as completely as possible and to achieve optimum adjustment, as many characteristics maps as possible are used, so that the reduction agent quantity can be accurately determined. Maximum nitrogen oxide conversion and minimum emission of unconverted reduction agent (reduction agent slip) is to be ensured in each operating state of the internal combustion engine and/or the catalytic converter, in particular at different temperatures, different injected fuel quantities, and/or different engine speeds. Prior to the initial startup of the engine and/or the catalytic converter, the characteristics maps must be recorded (calibrated) in advance, by the manufacturer, for example. The more characteristics maps are used, the greater is the metering accuracy during each operating state of the catalytic converter and/or each operating state of the combustion system, but also the greater is the calibration complexity and the more complex is the assignment of the characteristics maps.